Births and baptisms
We generally have only baptism records prior to 1837 in England and Wales and prior to 1855 in Ireland and Scotland. There are very few records of dates of birth.
When dealing with baptisms, bear in mind that they were not always conducted immediately after the birth. There was usually a slight delay, or a longer delay if the birth took place away from the mother’s home parish.
Two children baptised on the same day were not necessarily twins.
Civil registration
In England and Wales, civil registration of births, marriages and deaths was introduced in 1837. See Overlap between parish registers and civil registration records.
It took a while for the new system to be understood, so some parents continued to have children baptised and did not see the need to register the birth; others registered the births of children who had been born a few years before 1837.
Once things settled down, however, civil registration became widely accepted. Bear in mind that a birth could be registered within 30 days, so it might have taken place in the preceding quarter. Marriages were registered on the day of the wedding; deaths were registered before the burial, which might have taken place in the following quarter.
Double year dates
‘Double dates’ can be misleading, especially for inexperienced researchers. See Dates before 1751.